1. Field of the Invention
This invention relates to a method and apparatus for the continuous production of bags from thermoplastic film and is more particularly concerned with improvements in the method and apparatus for the continuous production of bags from a multi-layer continuous web of thermoplastic film in which the web is passed on to a rotating drum where successive portions are seam welded and severed in lengths corresponding to the width of the desired bags.
2. Description of the Prior Art
Various methods and apparatus are known in the art for the continuous production of bags such as garbage bags and the like from thermoplastic film. One such method, and apparatus for carrying out the method is that described in van der Meulen U.S. Pat. No. 4,023,470. The method comprises feeding a web of a two layer thermoplastic film, in which the layers are continuously joined along one longitudinal edge, tangentially on to a rotating welding drum under tension. The tension is achieved by employing web loop formers which engage the web at a location prior to its arrival o the drum. The tension serves to ensure that the web is drawn on to the drum surface in taut and smooth form thus enabling the web to be fed at high speeds. At the time of each seam welding and severing step the web tension is released momentarily by disengaging the web loop formers. Carrying out the welding on the unstressed web is said to ensure a better seam weld and avoid damage to the bag or weld which might occur if the web were under stress during the welding operation.
It has been found that the method described in the above patent can be employed satisfactorily and successfully in production of bags at relatively high speeds so long as the web of film layers employed has a relatively uniform thickness across the width of the web and so long as the thickness of each of the film layers does not exceed about 3 mils. When these conditions are not met it is found that problems arise. Thus where, for example, the free edges of the film layers are folded over to form a hem so that there are four thicknesses of film along these edges and only two thicknesses of film across the rest of the width of the web, it is found that satisfactory seam welding and severance cannot be achieved in the four film thick region in the time required to weld and sever the two film thick region. This problem is even more acute in the case of trash bags which are provided with a draw-string. In this instance the thickness of two draw-strings is added to the four thicknesses of film in the hems which accommodate the draw-strings to make a total thickness of 14 mils or even higher which has to be sealed and severed. If the time for seam welding and severance is increased to permit satisfactory completion of this step in the thicker region the seam formed in the thinner region may be affected deleteriously. Further the rate of production is reduced as a result of the increased time taken to accomplish the step. The latter is also the case where the thickness of each of the film layers, although uniform across the width thereof, exceeds of the order of 3 mils. Such films, which are required for heavy duty bags, require a significantly longer period in which to accomplish satisfactory seam welding and severance.
Attempts have been made to overcome these problems by carrying out a "preliminary seal" at the location on the web at which the ultimate seam welding and severance step is to take place. The terms "preliminary seal" and "preliminary sealing" as used herein and throughout this specification and claims mean a preliminary heating step applied at designated locations on the thermoplastic material of the web which step serves at least to soften the material at the location so treated. The term is not intended to imply that actual sealing of some or all of the various layers of the web takes place.
These disadvantages in the prior art methods are overcome by the method and apparatus of the present invention which will be described in detail below. Further, it has been found that the method of the present invention enables one to reduce significantly the width of the seam formed on the edges of the bag produced thereby. This is an important consideration in the case of bags such as trash bags provided with a drawstring closure. Thus the amount by which the bag can be opened is reduced by the width of the seams on either side of the bag. An reduction in the size of the opening makes is difficult to fit the bag mouth over a garbage can rim when the bag is used as liner. This reduction in width of seams is achieved by the present invention because of accurate registration with the preliminary seal in the final seam welding and severance step. Thus, it has been found that, using the process and apparatus of the present invention, preliminary seals having a width as low as about 0.125 inches in the direction of movement of the web can be achieved routinely and that the width of the side seam weld on the bags so produced is of the order of about 0.0625 inches. These findings represent a marked improvement over the results achieved using previous methods available in the art.
The various advantages discussed above and additional advantages which will become apparent from the description which follows will be readily understood by those skilled in the art.